Conventionally, a two-piece paper cup or similar container is made up from a more or less keystone-shaped side wall blank and a disc-like bottom wall blank. The side wall blank usually has downwardly convergent side edges, a concavely curved bottom edge, and a convexly curved top edge. Before the two blanks are assembled with one another, the marginal portion of the bottom wall blank is bent or crimped to form it into an axially projecting flange which is to be bonded to the side wall of the cup.
For assembly of the two blanks, a strip-like coating of glue is applied along one side edge of the side wall blank and along its curved bottom edge. The bottom wall blank is attached by suction to the small diameter end of a tapering mandrel, and the glue coated side wall blank is wrapped around the mandrel to bring its side edge portions into overlapping relation to one another for formation of a side seam. The coating of glue along the bottom edge of the side wall blank bonds that portion of it to the flange on the bottom wall blank and forms a bottom seam.
Heretofore there has been no completely satisfactory device for applying glue to paper cup side wall blanks. For high production, the glue applicator must be moved down into contact with each blank and immediately withdrawn upwardly from it; and during the brief instant of contact a thin but even coating of glue must be applied to the desired area of the blank.
One type of prior glue applicator comprised a body having a flat bottom surface and in which there was a narrow, elongated manifold passage which was communicated with a source of glue and from which restricted outlets opened downward to the bottom surface. Because of the viscosity of the high-tack glue needed for paper cup manufacture (usually of a polyvinyl acetate type) the glue was normally delivered to the glue applicator under pressure, and the glue outlets were in the form of narrow slots or small holes through which the glue issued onto the blank. Delivery of glue from the pressurized source to the glue applicator was valved in synchronism with the up and down movement of the applicator. Since glue issued from the outlets in a pattern of discrete dots or spots, enough glue had to be applied for some spreading of glue to take place, so that there would be a continuous film over the whole of the coated area, but of course an excessive application of glue was almost as undesirable as an insufficient one. The individual glue outlets had to be small enough for a capillary effect by which glue was prevented from dripping out of the applicator (although a certain amount of dripping was nearly inevitable), and therefore the size of the outlets was very critical.
With a new, clean applicator, just the right amount of glue could be applied at each operation, over exactly a desired area. However, any accidental denting of the applicator could result in a total or partial blockage of one or more of its outlets that would disturb the pattern of application. More significantly, any interruption of operations would allow glue to dry around the edges of the outlets and block them. Because of the small size of the outlets, cleaning them of dried glue was a time consuming and rather difficult task.
Other types of glue applicators that have been used in paper cup machines have had other disadvantages. In some cases, glue was applied by means of a roller applicator that was moved along the blank. Such an applicator was slow and required a complicated and expensive mechanism for effecting roller traverse. In other cases glue was applied to a plate, analogously to the application of ink to a printing press plate, and the plate was then brought into contact with the blank. Here, again, complex mechanism was needed.